The present invention pertains to an abrasive article and to a method of using the abrasive article to abrade a glass or glass ceramic workpiece.
Glass ceramics are known to be used as substrates for magnetic memory discs, for example, those used as storage devices (e.g., hard drives) in personal computers. In order to produce an acceptable magnetic memory disc, the memory disc substrate must have precisely controlled dimensions and a precisely controlled surface finish. Typically, dimensioning and imparting the desired surface finish to memory disc substrates has involved a multi-step process using loose abrasive slurries. In the first step of the process, the glass ceramic discs are dimensioned such that they have the desired thickness and thickness uniformity. After dimensioning, the discs are textured to provide the desired surface finish.
Although loose abrasive slurries are widely used in this process, loose abrasive slurries have many disadvantages associated with them. These disadvantages include the inconvenience of handling the required large volumes of the slurry, the required agitation to prevent settling of the abrasive particles and to assure a uniform concentration of abrasive particles at the polishing interface, and the need for additional equipment to prepare, handle, and dispose of or recover and recycle the loose abrasive slurry. Additionally, the slurry itself must be periodically analyzed to assure its quality and dispersion stability. Furthermore, pump heads, valves, feed lines, grinding laps, and other parts of the slurry supply equipment which contact the loose abrasive slurry eventually show undesirable wear. Further, the processes which use the slurry are usually very untidy because the loose abrasive slurry, which is a viscous liquid, splatters easily and is difficult to contain.
In view of the many disadvantages associated with using a slurry process to abrade (i.e., dimension or texture) glass ceramic memory disc substrates, what is desired in the industry is a fixed abrasive article suitable for abrading these substrates.
The present invention provides an abrasive article which is suitable for abrading (i.e., dimensioning or polishing) a glass or a glass ceramic workpiece. The abrasive article comprises a backing and at least one three-dimensional abrasive coating bonded to a surface of the backing. The abrasive coating comprises a binder having dispersed therein a plurality of diamond bead abrasive particles and a filler. The filler comprises about 40 to about 60 percent weight of the abrasive coating, more preferably about 50 to about 60 percent weight of the abrasive coating.
It is preferred that the three-dimensional abrasive coating includes a plurality of abrasive composites. The plurality of abrasive composites can be precisely shaped composites, irregularly shaped composites or precisely shaped composites including a shape of a truncated pyramid having a flat top. Preferably, the precisely shaped composites have a bottom portion defining a surface area not more than 50%, more preferably, not more than 25% and most preferably, not more than 15% greater than the top surface area of the composites.
Preferably, the binder is formed from a binder precursor comprising an ethylenically unsaturated resin, for example, an acrylate resin. The ethylenically unsaturated monomer preferably is selected from the group of monofunctional acrylate monomers, difunctional acrylate monomers, trifunctional acrylate monomers, and mixtures thereof
The abrasive particles in an abrasive article of the present invention comprise diamond bead abrasive particles. The diamond beads comprise a plurality of individual diamond particles which are held together by a metal oxide matrix, preferably a silicon oxide matrix. Preferably, the average size of the diamond bead abrasive particles is about 6 to about 100 micrometers.
Abrasive articles of the present invention have been found to be particularly suitable for abrading glass and glass ceramic workpieces. That is, abrasive articles of the present invention provide a high cut rate which is relatively constant over the life of the abrasive article when they are used with a lubricant to abrade a glass or a glass ceramic workpiece. Therefore, another aspect of the invention is directed to a method for abrading a glass or a glass ceramic workpiece comprising the steps of:
(a) contacting a glass or a glass ceramic workpiece with an abrasive article as described above;
(b) applying a lubricant at an interface between the workpiece and the abrasive article; and
(c) moving the workpiece and the abrasive article relative to one another.
The three dimensional abrasive coating of the abrasive article comprises a binder having dispersed therein diamond bead abrasive particles and at least one filler in the amount of about 40 to about 60 percent weight of the abrasive coating. The level of filler is chosen to provide an abrasive coating which will erode under typically use conditions thereby exposing and releasing new diamond bead abrasive particles. Diamond bead abrasive particles are believed to be particularly suitable because their relatively large size inhibits them from being pressed into the abrasive coating. Also, it is believed that the diamond bead abrasive particles are less susceptible to developing wear flats (i.e., less susceptible to dulling) which may lead to a reduced cut rate.
In a preferred embodiment, the three dimensional abrasive coating has a precisely shaped surface. xe2x80x9cPrecisely shapedxe2x80x9d as used herein, describes the abrasive composites which are formed by curing the binder precursor while the precursor is both being formed on a backing and filling a cavity on the surface of a production tool. These abrasive composites have a three dimensional shape that is defined by relatively smooth-surfaced sides that are bounded and joined by well-defined sharp edges having distinct edge lengths with distinct endpoints defined by the intersections of the various sides. This type of abrasive article is referred to as structured in the sense of the deployment of a plurality of such precisely-shaped abrasive. The abrasive composites may also have a irregular shape which, as used herein, means that the sides or boundaries forming the abrasive composite are slumped and not precise. In an irregularly shaped abrasive composite, the abrasive slurry is first formed into the desired shape and/or pattern. Once the abrasive slurry is formed, the binder precursor in the abrasive slurry is cured or solidified. There is generally a time gap between forming the shape and curing the binder precursor. During this time gap, the abrasive slurry will flow and/or slump, thereby causing some distortion in the formed shape. The abrasive composites can also vary in size, pitch, or shape in a single abrasive article, as described in WO 95/07797 (published Mar. 23, 1995) and WO 95/22436 (published Aug. 24, 1995).
xe2x80x9cBoundaryxe2x80x9d as used herein, refers to the exposed surfaces and edges of each composite that delimit and define the actual three-dimensional shape of each abrasive composite. These boundaries are readily visible and discernible when a cross-section of an abrasive article of this invention is viewed under a microscope. These boundaries separate and distinguish one abrasive composite from another even if the composites abut each other along a common border at their bases. For precisely shaped abrasive composites, the boundaries and edges are sharp and distinct. By comparison, in an abrasive article that does not have precisely shaped composites, the boundaries and edges are not definitive (i.e., the abrasive composite sags before completion of its curing). These abrasive composites, whether precisely or irregularly shaped, can be of any geometrical shape defined by a substantially distinct and discernible boundary, wherein the precise geometrical shape is selected from the group consisting of cubic, prismatic, conical, block-like truncated conical, pyramidal, truncated pyramidal, cylindrical, hemispherical and the like.
xe2x80x9cTexturexe2x80x9d as used herein, refers to an abrasive coating having any of the aforementioned three dimensional composites, whether the individual three dimensional composites are precisely or irregularly shaped. The texture may be formed from a plurality of abrasive composites which all have substantially the same geometrical shape (i.e., the texture may be regular). Similarly, the texture may be in a random pattern where the geometrical shape differs from abrasive composite to abrasive composite.